1. Field of the Invention
The present invention relates to stretch wrapping machines having prestretchers for elongating a stretchable wrapping material from its initial length as unwound from a supply roll to a controlled elongation prior to wrapping of a load located on a rotatable turntable. More particularly, the invention relates to prestretchers which utilize a single power driven roller to achieve prestretching.
2. Description of the Prior Art
French Pat. No. 2,281,275 to Thimon discloses a prestretching system which utilizes a pair of power driven rollers to achieve prestretching of a stretchable wrapping material prior to the wrapping of a load located on a rotatable load support. In this patent it is taught that the speed of the downstream roller is greater than the upstream roller located closest to the supply roll to achieve stretching of the stretchable wrapping material. The relative velocities of the rollers controls the amount of prestretching.
U.S. Pat. No. 4,418,510 discloses a process similar to that disclosed in Thimon. A pair of driven rollers function as a prestretcher for elongating the stretch wrapping material prior to wrapping around a load disposed on a rotatable load support. The degree of elongation is controlled by the relative peripheral velocities of the pair of rollers. U.S. Pat. No. 4,418,510 also discloses that the combination of a single power driven roller and a braked film supply roll may be used to perform prestretching.
Stretching of stretchable wrapping material between the load to be wrapped and the supply roll has also been accomplished by the use of a friction brake for varying the degree of force required to unwind the stretch wrapping material during wrapping of the load which controls the degree of stretch achieved from the nominal dimension of the stretchable wrapping material as wound on the supply roll. However, conventional friction brakes are difficult to control when objects with corners are being wrapped because of the cyclical variation of the wrapping speed of the stretchable wrapping material around the object as a consequence of corner passage.